Coals contain sulfur compounds, both as inorganic materials such as pyrites, and as complex organic materials. During combustion of the coal, the sulfur is oxidized to sulfur oxide, noxious gasses that are highly corrosive to furnace equipment and which constitute health hazards in relatively small amounts. Because of the latter property, the emission of sulfur oxide is severely limited by environmental regulation. This limitation on the emission of sulfur oxides coupled with the expense and difficulty in scrubbing these materials from the combustion furnace stack gasses puts an economic premium on coals containing low sulfur levels.
Techniques are presently available for removing the inorganic fractions of the sulfur compounds from coal either by chemical reaction or by physical separation. Such processes frequently involve additional grinding of the coal to a fine particle size to make the pyrites more available and water washing to some degree. Either of these sulfur removal steps requires additional treatment to the coal beyond that which is usually used and thus adds to the overall cost.
Pertinent prior art includes a paper given at the American Chemical Society Symposium, August 29-31, 1972 in New York, New York: "Engineering, Economic and Pollution Control Assessment of Meyers' Process for Removal of Pyritic Sulfur from Coal", and the Bureau of Mines Report of Investigations 7633, "Sulfur Reduction Potential of Coals in the United States", A. W. Deuebrovck, Pittsburgh Energy Research Center. U.S. Pat. No. 2,128,913 discloses the mixing of air with a coal slurry prior to its introduction into a pipeline, and U.S. Pat. No. 2,346,151 discloses the addition of air to a coal slurry, in this case for the separation of undersized particles by flotation. Another patent of less pertinence is U.S. Pat. No. 3,768,988.